In analog telephones, an upper limit frequency of the frequency band of a voice signal sent via a telephone line is band-limited narrowly to 3,400 Hz, for example. Therefore, it cannot be said that the sound quality of a conventional telephone line is good. Further, because the bandwidth is limited by a severe restriction on the bit rate in the case of digital speech communications, such as mobile phone communications and wireless communications for commercial use, like in the case of analog channels, also in this case, it cannot be said that the sound quality is good. Although a voice consonant component and a voice's own “individual nature (person likeness)” and “naturalness” often exist also in a band equal to or higher than 3,400 Hz, they are greatly lost due to the above-mentioned bandwidth limitation.
By the way, in recent years, it has become possible to carry out radio transmission of a voice signal in a wide band (e.g., an upper limit frequency is 7,000 Hz) at a low bit rate as audio compression technologies (voice coding techniques) move forward. However, because both a transmitting side terminal and a receiving side terminal need to support a corresponding wideband voice encoding method and a corresponding wideband voice decoding method, respectively, and both base stations need to be also equipped fully with a network for wide band encoding, such radio transmission is put to practical use only in some communication systems for commercial use, and, in the case of implementing such radio transmission in a public telephone communication network, not only a large economic burden is imposed, but also it takes much time before it is widely available.
To solve this problem, a trial of compressing the frequency of a voice signal to make an entire spectrum of the voice signal fall within a passband has been performed. A problem with this method is, however, that while the voice signal including signals in a lower frequency band including voice fundamental periods (pitches) is compressed to a lower frequency range, as compared with the original voice signal, when this compressed signal is reproduced without being expanded, the reproduced voice has a low pitch unnaturally, and lacks the individual nature and its quality degrades remarkably. In contrast with this, although this problem can be solved by expanding the compressed signal in the receiving side terminal, in order to implement this expansion, the receiving side terminal needs to support an operation of expanding a high band signal compressed by the transmitting side terminal. Like in the case of supporting band broadening, such a solution is not practical for a communication terminal which is not ready for transmission of compressed information together with a voice signal, and expansion of the compressed information.
For the above-mentioned problem, for example, there has been provided a technique described in patent reference 1. In this technique, by using a method of compressing only a predetermined high-frequency region without changing the voice pitch, a clear voice signal is acquired.